The present invention relates to a gas supply system. More particularly, the present invention is directed to the supply of ultra high purity gases in large volumes and at high flow rates from a container of liquefied gas.
The growth of electronic and fiber-optic industries has created a demand for a supply of large quantities of ultra high purity (UHP) gases. Historically, UHP gases were shipped to consumers in cylinders, Y-cylinders (see discussion below), and toners. The increasing demand for UHP gases has shown that use of small and mid-size vessels is no longer adequate. Therefore, large vessels such as tube trailers, ISO (International Standards Organization) containers, tankers, and the like, are considered more viable.
ISO containers have long been a standard vehicle for transporting equipment and other goods via air, land, sea, and rail. These containers are durable, rugged in construction, and are sized and shaped such that they are readily and economically securable to rail cars, trucks, ship holds, and cargo bay floors of large aircraft. These freight containers are of standard dimensions, and are used in international transport whether by land, sea or air. Additionally, these containers are provided with corner fittings which may be used both to lift the container, and also to lock it to a vehicle on which it is being transported. The dimensions of these containers are laid down by the International Organisation for Standardisation, and they are accordingly referred to as ISO containers.
The purity of the delivered gases is the most critical factor of the bulk gas delivery system. UHP gases must meet very stringent specifications for moisture, metal content, particles, and the like. For example, 1 part per million (ppm) moisture content in the gas phase is often considered to be the maximum moisture level permissible for a gas used in high technology industries. The problem with bulk UHP gas delivery systems is enhanced by the fact that there is little experience in the industry in the use and preparation of large size containers.
Typically, a UHP gas delivery system is divided into two major parts. The first part is a vessel, which stores and delivers a liquefied gas. The second part is vaporizer, which vaporizes liquid, supplying the gas phase to a distribution system. Each part of the described gas delivery system is independent from the other. As noted above, a major concern associated with such a system is gas purity. Vaporizers may become an additional source for gas contamination. In addition, vaporizers typically take a lot of space and may be quite costly.
One attempt made to eliminate a vaporizer and to deliver the gas phase directly from the vessel is described in U.S. Pat. No. 6,025,576 (Beck et al.) for a bulk vessel heater skid for liquefied compressed gases. This patent addresses a problem where compressed gases are dispensed from cylinders, as follows. As the high pressure gases are emitted from the cylinder, the expansion of the gases absorbs thermal energy which causes a cooling at the point of dispensation that propagates throughout the cylinder to cause an undesirable cooling of the cylinder walls and of the gases within the cylinder. Cooling at the valve or regulator can cause frosting that creates other problems with gas flow in the overall system. Where the gases are compressed and liquefied within the cylinder, the evaporation of liquid to gas also causes cooling of the liquid, gas and cylinder. This causes the cylinder pressure (vapor pressure) to drop. The effect of the cooling is to reduce the maximum steady state flowrate that can be obtained from the cylinder. Extremely low temperatures can be created which can cause xe2x80x9cembrittlementxe2x80x9d of the cylinder that can result in a rupture and uncontrolled energy release from the highly pressurized cylinder. Moreover, such an energy release may be associated with flammable or combustible products.
The trend in industry is to require higher gas flow rates from larger cylinders which increases the cooling problems. By using larger cylinders of liquefied compressed gases, the supporting and maintenance of numerous small cylinders is eliminated and space is conserved. These larger cylinders are called xe2x80x9cbulk vesselsxe2x80x9d or xe2x80x9ctonnage containers.xe2x80x9d In particular, U.S. Pat. No. 6,025,576 addresses a popular type of bulk vessel such as the xe2x80x9cYxe2x80x9d cylinder. The xe2x80x9cYxe2x80x9d cylinder is approximately 24 inches in diameter by approximately 7 feet long and weighs about 1150 lbs., empty. Chemicals such as HCl and ammonia are commonly dispensed in bulk gas delivery systems using the xe2x80x9cYxe2x80x9d cylinder. While the current demand is for gas flows in the range of 100-500 standard liters per minute (slpm), it is difficult to provide a rate higher than about 25 slpm for some gases because of the adverse effects from cooling in bulk gas delivery systems using the xe2x80x9cYxe2x80x9d cylinder.
Various measures exist in the prior art for trying to maintain the temperature of a dispensing cylinder. One approach is to cover the cylinder in a thermal insulation material which helps to sustain the temperature of the cylinder. However, merely using insulation does not keep the cylinder at sufficiently high temperatures and may actually prevent ambient heat from heating the cylinder.
More effective is the use of heaters applied to the cylinder to alleviate the cooling effect resulting from the dispensing of gas. However, in the past, the cylinders were handled and stored by placement or attachment to skeletal frameworks, or xe2x80x9cskids.xe2x80x9d This made it time consuming and cumbersome to attach heaters to the cylinder. Many of the transport skids provided little room to secure the heaters. The heaters must be attached when the cylinders are taken from a transport skid and placed onto a dispensing skid. The heaters must later be removed when the cylinder is exhausted and needs to be sent back for re-filling.
U.S. Pat. No. 6,025,576 teaches a skid with built in heating elements for heating and supporting a compressed-gas dispensing bulk vessel. A disadvantage of the system of U.S. Pat. No. 6,025,576 is that it has two substantial elements, the vessel and a separate heater skid. While this system may be applicable for mid-size cylinders such as Y-containers or toners, this system cannot feasibly be used for bigger vessels, such as ISO containers. If used with an ISO container, the skid would have a substantial weight if mounted together with the ISO container. This will reduce the container size to comply with transportation requirements. On the other hand, the ISO container cannot be placed on the skid, which is used as a stand-alone unit, due to the container frame structure. Therefore, a different system is needed.
An ideal system would satisfy the following requirements. First, the container should contain large quantities of liquefied gas (e.g., more than 2,000 lbs and up to about 20,000-50,000 lbs). Second, the system should be transportable around the world. Third, the system should have simple, safe, and easy connections when at an loading/unloading site. Fourth, the system should be capable of delivering high flow rates of UHP gases.
The present system addresses these requirements.
The present invention is directed to a high flow rate, transportable, ultra high purity gas vaporization and supply system. The system includes a vessel suitable for carrying large quantities of a liquefied gas, a plurality of valves adapted to operate with liquid or gas phases, a loading/unloading unit disposed on the vessel for loading and unloading the liquefied gas to be supplied, and a heater containing heating elements permanently positioned on the vessel to supply energy into the liquefied gas. The heater causes the liquefied gas to be supplied through the loading/unloading unit as a gas. A heater controller is also provided which uses process variables feedback for regulating the heating elements to maintain and regulate gas output.
The vessel is preferably an ISO container, tube trailer, or tanker. The vessel is suitable for carrying over about 2,000 lbs. and up to about 20,000 to 50,000 lbs. of the liquefied gas. Preferably, the vessel is covered with thermal insulation. The heating elements may be divided into heating zones. The heater controller preferably utilizes temperature measurement elements to provide feedback to the heater controller and preferably includes a programmable logic controller to stagger activation of the heating elements. The heating elements are preferably connected to the heater controller utilizing quick-connect electrical plug assemblies to permit replacement of an empty vessel with minimal effort. The system preferably includes high temperature switches associated with the heating elements, wherein the switch includes a temperature set point where the switch disconnects associated heating elements when the set point is reached. The heating elements may be grouped into heating zones that are separately controlled by the heater controller. A ground-current leakage monitor that automatically disconnects power to the heating elements when leakage current exceeds a predetermined value, for example, 100 mA may be included. An over current limit device that automatically disconnects power to at least some heating elements when current exceeds a predetermined value may also be included. The heating elements are preferably located so as to minimize direct heating above the lowest expected vapor-liquid interface level. By doing so, gas phase purity is maximized.
A method for providing high flow rate, transportable, ultra high purity gas is also provided which includes providing the above system and then controlling flow of the gas out of the vessel through the loading/unloading unit by the heater controller utilizing process variables feedback to regulate the heating elements.